A wide variety of organic and inorganic compounds are used for the complex series of steps by which a negative-working silver halide emulsion of high sensitivity is produced. One such step involves the chemical sensitization of the silver halide grains to increase their light sensitivity. Between the time of adding the sensitizer and coating the liquid emulsion, the latter is usually given a heat treatment, called digestion. During digestion a reaction is believed to occur which produces sensitivity sites on the surface of the silver halide grains. Unfortunately, as the digestion reaction is continued in order to obtain a higher level of sensitivity, some silver halide grains become spontaneously developable without exposure. This causes the emulsion to fog. Films made with grains which have undergone digestion to achieve high sensitivity not only exhibit this fog when tested shortly after being coated, but display higher levels of fog as the film is aged. This may reach a level such that the film is unusable and in any case limits the useful life of the film. Undesirable losses in sensitivity may also accompany the increase in fog as the film ages.
Efforts to obtain higher sensitivity for negative-working silver halide emulsions must in some fashion deal with the problem. An overall balance must be established between sensitizing effects which promote high speed and the associated tendency for fog, and stabilizing effects which lower the fog but tend to also lower speed.
It has been a common practice in the photographic art to characterize many of the additions made in the process as either predigestion additions or post digestion or after-additions. Gold compounds such as gold chloride and gold thiocyanate and sulfur compounds such as the thiosulfate and thiourea are well known in the art as predigestion additions for chemical sensitization (James, "The Theory of the Photographic Process," 3d Ed., pgs. 113-116 (1966)). They provide high speed emulsions, after a suitable digestion period; both inorganic and organic compounds are added to stabilize the emulsion. Such after-additions also lower the fog level, presumably by undoing some of the sensitization achieved by predigestion additions.
U.S. Pat. No. 4,323,645 discloses that certain organic halogen compounds such as p-nitrobenzyl chloride give reduced fog levels accompanied by little or no speed loss when added as after-additions, or even when in-line injected into the emulsion during the coating step to produce a photographic film. The present invention improves on the teachings of that patent by adding the same compounds to a silver halide emulsion to reduce emulsion fog, but is directed to the discovery that a much lower amount, if added earlier in the process, is equally effective.